1. Field of the Invention
The present invention relates to a liquid-crystal display, particularly to a liquid-crystal display in which a potential of an alignment film is controlled.
2. Discussion of Related Art
Liquid-crystal displays of various modes have been developed so far. For example, there are TN (Twisted Nematic) mode and VA (Vertical Alignment) mode. The TN mode seals a nematic liquid crystal having a positive dielectric anisotropy between two glass substrates and twists an arrangement of liquid crystals by using an alignment film formed on both the substrates. By applying an electric field to the liquid crystals, the arrangement of the liquid crystals is controlled in the vertical direction to perform screen display by using polarization of transmitted light. The VA mode arranges liquid crystals having negative dielectric anisotropy in a direction vertical to a glass substrate. Transmission of light is controlled by applying an electric field to the liquid crystals and thereby tilting the direction of the liquid crystals.
In addition to liquid-crystal displays having the above modes, a liquid-crystal display is known which has a mode referred to as IPS (In Plane Switching) developed to improve a viewing angle. The liquid-crystal display controls transmission of light by operating a direction of a liquid crystal in a plane parallel with a glass substrate and has a structure in which electrodes for applying an electric field to liquid crystals are formed on the same substrate. FIG. 7 shows a conventional IPS-mode TFT liquid-crystal display, which is an illustration showing a configuration of one of sub-pixel portions arranged like a matrix in a display pixel area on a TFT array substrate. In FIG. 7, reference numeral 702 denotes a plurality of pairs of common wirings extending in one direction in parallel with each other and 703 denotes gate wirings extending in one direction in parallel with each other and arranged in parallel with the common wirings.
Reference numeral 704 denotes a plurality of source wirings extending in one direction in parallel with each other and arranged in a direction almost orthogonal to the common wiring 702 and the gate wiring 703. Reference numeral 705 denotes a sub-pixel portion that is enclosed by the common wiring 702, gate wiring 703, and source wiring 704. Reference numeral 706 denotes a TFT serving as a switching device. The TFT 706 is constituted by arranging a source electrode 707 extending along the gate wiring 703 from the source wiring 704 and a drain electrode 708 in parallel with each other at both the sides of a constant channel 709. The gate wiring 703 is formed below the channel 709 formed of a semiconductor layer through an insulating film (not illustrated).
Two common electrodes 710 extend downward in FIG. 7 from the common wiring 702 along two source wirings 704 while the drain electrode 708 of the TFT 706 is connected to a pixel electrode 711. The pixel electrode 711 extends upward in parallel with these two common electrodes 710 between two common electrodes 710. Moreover, a plurality of pairs of parallel electrodes are formed in one pixel and contrast of pixels is produced by controlling an orientation of liquid crystals in accordance with the intensity of an electric field between these electrodes to provide a display screen. Some of electric charges supplied to the pixel electrode 711 from the source wiring 704 through the TFT 706 are held by a storage capacitance 712.
Though not shown in FIG. 7, it is needless to say that similarly to an ordinary TFT liquid-crystal display an alignment film is formed on a TFT array substrate having the above configuration and surfaces of counter substrates arranged in parallel with each other separately from the array substrate by a predetermined gap and the gap between these two substrates is filled with liquid crystal. Moreover, the portion shown in FIG. 7 shows a pixel portion of each of R, G, and B constituting one pixel when performing color displaying.
FIG. 8 is a sectional view showing a configuration of a liquid-crystal cell of a conventional IPS-mode TFT liquid-crystal display. Reference numeral 801 denotes a liquid-crystal cell having a function for displaying video information on a screen as the information of light transmittance of the pixel 705 by electrooptically converting an electrical signal including the video information input from a driving circuit. Reference numeral 802 denotes a TFT array substrate. A signal input from a driving circuit is distributed to the pixel electrode 711 through the gate wiring 703, source wiring 704, and TFT 706. The sub-pixels 705 are arranged in a display pixel area 817 like a matrix. A common potential is distributed to the common electrode 710 in the pixel 705 through a common-potential supply wiring 819 and the common wiring 702.
A gate insulating film 804 and a passivation film 805 are formed on the TFT array substrate 802. Reference numeral 803 denotes a counter substrate. The following are formed on the counter substrate 803: a black matrix 807 for shading a boundary portion of the pixel 705 and a display-screen circumferential area 818, a color filter 808 for dividing light into three primary colors of R, G, and B, and a protective film 809. The TFT array substrate 802 and the counter substrate 803 are arranged in parallel with each other while keeping a predetermined gap between them and liquid crystal 810 is sealed between them. An alignment film 806 for determining the initial orientation of the liquid crystal 810 is formed on faced surfaces of two substrates. Moreover, a polarization film 811 is formed on the outside surfaces of two substrates.
Because potentials different from each other are supplied to the common electrode 710 and the pixel electrode 711, an electric field is generated between the two electrodes, the electric field works on the liquid crystal 810 whose initial orientation is determined by the alignment film 806 in a direction different from the direction of the electric field and orientations of the liquid crystal are changed. In this case, the way of change of orientations of liquid crystal depends on the intensity of an electric field. When polarized light obtained after the light emitted from the backlight 812 passes through the polarization film 811, passes through the layer of the liquid crystal 810, intensities of the light can be changed due to orientation change of liquid crystal. Therefore, it is possible to change intensities of the light emitted from the other polarization film 811. Thereby, it is possible to change the information of an electrical signal to the information of intensity of light.
In the case of the conventional IPS-mode TFT liquid-crystal display shown in FIGS. 7 and 8, an alignment film is electrically floated as an insulating film is present between two electrodes and the alignment film in the display pixel area on the TFT array substrate. The two electrodes are for supplying an electric field to liquid crystal, that is, a pixel electrode and a common electrode. The alignment film is for providing initial orientation for liquid-crystal molecules. Moreover, because only materials having a high electric resistance are formed on a counter substrate, an alignment film formed on the counter substrate is also electrically floated. In the case of the above electrically insulated alignment film, there is a deviation in the direction of an electric field passing through an alignment film. Thereby, impurity ions corresponding to the direction of the electric field are easily collected on the film. Because ions are originally easily adsorbed by an alignment film, a screen display trouble such as an after-image or image-sticking occurs due to collected impurity ions.
Moreover, in a circumferential area nearby a display pixel area, particularly an area for extending a gate wiring to a driving circuit, the gate-wiring density is high compared to other areas. Therefore, because a gate-wiring potential whose average potential is approx. 10V lower than the average potential in a screen is predominant, an electric field passing through an alignment film is large and deviated. Thus, it is newly found that impurity ions are easily collected on an alignment film formed through an insulating film thereon. Therefore, the electric-charge holding characteristic of a pixel is easily deteriorated along the circumference of a display screen and this causes display blur or the like. This area has the same display blur problem because an alignment film on a TFT array substrate is electrically insulated in the case of not only an IPS-mode TFT liquid-crystal display but also TN-mode and VA-mode TFT liquid-crystal displays.
To solve the above problem, it is disclosed in Published Unexamined Patent Application No. 10-301141 to make an alignment film directly contact with an electrode wiring without forming an insulating film between them. In the case of the above invention, however, because a pixel electrode and a common electrode are electrically connected with an alignment film in a pixel, a large problem occurs if there is a defect such as disconnection or short circuit in the pixel electrode. Because a gate potential is supplied to the alignment film when the pixel electrode connects with a gate wiring. Moreover, this example does not disclose a method for preventing image-sticking or display blur in a display circumferential area at all.
Moreover, it is disclosed in Published Unexamined Patent Application No. 4-359222 to form a hole on an alignment film and an insulating film and electrically connect an electrode with liquid crystal. However, this invention does not relate to an active-matrix-type liquid-crystal display or does not disclose the knowledge on the above problem at all.
The present invention is made to solve the above problems and its object is to provide a liquid-crystal display for realizing a high-quality and high-reliability screen display. It is another object of the present invention to provide a liquid-crystal display capable of controlling image-sticking and display blur.
It is still another object of the present invention to provide a liquid-crystal display capable of preventing defects due to impurity ions in an alignment film. It is still another object of the present invention to provide a liquid-crystal display capable of preventing defects due to impurity ions in an alignment film without being influenced by a defect such as disconnection or short circuit in a pixel electrode. It is still another object of the present invention to provide a liquid-crystal display capable of preventing defects due to impurity ions in a circumferential area. It is still another object of the present invention to provide a liquid-crystal display capable of efficiently supplying a common potential to an alignment film.
A liquid-crystal display of the present invention supplies a common potential to an alignment film in a sub-pixel portion, preferably forms an opening on an insulating film formed between an alignment film and a common electrode wiring and electrically connects the alignment film with the common electrode wiring through the opening. A pixel electrode wiring is insulated from the alignment film in the sub-pixel portion.
Preferably, an opening is formed between a common wiring and an alignment film or between a common electrode wiring and an alignment film. Preferably, an alignment film is connected with a common wiring through a conductor portion and more preferably, the conductor portion is formed by exceeding the common electrode wiring and the alignment film is electrically connected with the conductor portion by contacting the conductor portion.
In the case of a liquid-crystal display of the present invention, a common potential is supplied to an alignment film in a circumferential area formed on the circumference of a display screen area. The alignment film is insulated from a gate wiring in the circumferential area. Preferably, the alignment film is electrically connected to a common wiring through an opening formed on an insulating layer.
Further preferably, the alignment film is connected with the common wiring through a conductor portion. The conductor portion is formed by exceeding the common wiring and the alignment film is electrically connected with the conductor portion by contacting the conductor portion. The liquid-crystal display includes display units such as a liquid-crystal cell, liquid-crystal module, and liquid-crystal display.